The invention relates to fluidized patient support systems, and more particularly, to fluidization level sensors and control systems for use with fluidized patient support systems.
Conventional patient support surfaces comprise a mattress-spring support layer with appropriate bed clothing and covering. Patients are sometimes immobile for long periods of convalescence, due to unconsciousness, coma, or paralysis, or are in a condition where movement is extremely painful. Prolonged patient contact with these conventional surfaces result in pressure points developing between the support surface and the patient""s body. Decubitus ulcers often develop on the patient""s skin at these pressure points, which further impedes the patient""s recovery and require additional medical treatment. Furthermore, conventional patient support surfaces are not conducive to burn patients with burns to significant portions of their bodies. In addition to developing decubitus ulcers, burn patients are at risk of developing infection from fluids exuding from the burn wounds. Over a relatively short time, conventional patient support surfaces become saturated with this excess fluid. Additionally, since the burn wounds are extremely sensitive to any contact, conventional patient support surfaces cause much discomfort to the patient.
Fluidized patient support surfaces overcome many of the problems inherent in conventional patient support surfaces. A fluidized patient support surface typically comprises an open tank filled with a fine granular material, such as fine glass or ceramic beads. The granular material is covered with a fluid permeable sheet, upon which the patient rests. The bottom of the tank is a diffuser surface through which a compressor forces a fluid, typically air. The fluid flows through the granular material, causing motion within the granular material.
In a non-fluidized state, the granular material has a specific gravity higher than water; however, when the granular material is fluidized, the specific gravity of the granular material is reduced significantly, and approaches a specific gravity near, but still greater than, the specific gravity of water. Thus, the fluid flow is adjusted so that the granular material behaves in a fluid like fashion, with the fluid permeable sheet providing a pressure release surface. Accordingly, the fluidized patient support imparts gentle forces on the patient""s body, and reduces the likelihood of developing decubitus ulcers. Furthermore, the fluid like behavior of the granular material provides a much more comfortable resting surface for burn patients.
The compressor mechanism of a fluidized patient support surface is adjusted to prevent over-fluidization and under-fluidization of the granular material. Over fluidization occurs when the fluid flow causes excessive turbulence in the granular material, which in turn creates a turbulent, boiling-like patient support surface that is uncomfortable and will also cause excessive heating of the granular material. Conversely, under-fluidization occurs when the fluid flow causes very little turbulence in the granular material, resulting in a hard patient support surface. Accordingly, the compressor mechanism is adjusted to ensure proper fluidization of the granular material.
A common problem occurring in fluidized patient support surfaces is the gradual wearing of the granular material. The surfaces of the granular particles become worn due to the abrasive action of the granular material motion. As these surfaces become worn, the granular material is less responsive to the fluid flowing through the diffuser, and thus the patient support surface tends to become under-fluidized. The compressor must then be adjusted to force more fluid through the diffuser to ensure that the patient support surface remains properly fluidized. The adjustment must also not be of such magnitude that the patient support surface becomes over fluidized. Thus, there is need for a fluidization control apparatus to maintain a substantially constant fluidization level independent of the wearing of the granular material.
According to the invention, a method and apparatus for sensing the fluidization level and controlling the fluidization level of a mass of granular material is provided. A fluidization sensing apparatus comprises a fluidization level sensor that outputs a signal proportional to the fluidization level of the granular material, and/or to the motion of the mass of granular material.
In one embodiment, the invention provides a fluidized patient support surface which includes a mass of granular particles, a compressor, a sensor, and a controller. The compressor produces fluid flow through the mass of granular particles causing the particles to fluidize. The sensor measures a fluidization level of the mass of granular particles, and produces a signal proportional to the fluidization level. The controller is coupled to the sensor, and receives a signal from the sensor and generates a control signal for controlling the level of fluid flow through the mass of granular particles.
In one embodiment, the control signal controls the level of fluid flow through the mass of granular particles so as to maintain a substantially constant fluidization level. The controller is preferably a proportional-integral (PI) controller. One embodiment of the controller comprises a high-pass filter coupled to an output of the sensor to remove low frequency noise, a peak detector having an input coupled to an output of the high-pass filter, and an integrator coupled to an output of the peak detector.
A variety of sensors may be used, including acoustic and infrared transducers. In one embodiment, an acoustic transducer in contact with the mass of granular material is employed. One infrared sensor, according to the present invention, comprises an emitter configured to emit an infrared signal, and a receiver configured to receive the infrared signal emitted by the emitter. The infrared sensor may be enclosed within a housing having a transparent side in contact with the mass of granular particles. The emitter is configured to emit an infrared signal through the transparent side, and the receiver is configured to receive the infrared signal and generate the signal proportional to the fluidization level. The transparent side may be formed from a material which serves as an optical filter and which is resistant to abrasion. One material from which the transparent side may be formed is a sapphire crystal. In certain embodiments, the housing includes a second transparent side, with the receiver being disposed to receive the infrared signal through the second transparent side.
One embodiment of the invention may also include an alarm indicator that is actuated when the signal proportional to the fluidization level or the control signal exceeds a threshold value. The alarm indicator may also be actuated when either signal falls below a predetermined threshold value.
In one embodiment, the invention further comprises a diffuser and a frame wall for confining at least a portion of the mass of granular particles. In certain instances, the sensor may be mounted on the frame wall.
Another aspect of the present invention provides apparatus for controlling the fluidization level of a mass of granular particles in a fluidized patient support system. The apparatus comprises a compressor in fluid communication with the mass of granular particles. The compressor is responsive to a compressor control signal and is configured to communicate a fluid through the mass of granular particles. The apparatus further comprises a sensor configured to output a fluidization control signal proportional to a fluidization level of the mass of granular particles. The apparatus further comprises a controller coupled to the output of the sensor and to the compressor. The controller generates a compressor control signal in response to the fluidization control signal to maintain a substantially constant fluidization level of the mass of granular particles.
Yet another aspect of the present invention is a method of controlling a fluidization level of a mass of granular particles in a fluidized patient support surface. The subject method comprises the steps of: (a) providing a controllable source of fluid to fluidize the mass of granular particles; (b) sensing the fluidization level of the mass of granular particles; (c) generating a control signal proportional to the fluidization level of the mass of granular particles; and (d) applying the control signal to a controller to adjust the source of fluid so as to achieve a desired level of fluidization. The sensing step of the method may further include providing an acoustic sensor within the mass of granular particles, or mounted to a wall or other surface adjacent the mass of granular particles. The sensing step may also include transmitting energy through at least a portion of the mass of granular particles, and receiving at least a portion of the transmitted energy as modulated by motion of the mass of granulated particles. The sensing step may further include mounting a transmitter and receiver adjacent a transparent side or sides of a housing disposed adjacent the mass of granular particles. The step of generating a control signal proportional to the fluidization level may comprise the steps of filtering an output signal produced by the sensing step, and conditioning the output signal through a peak detector.
Additional features, attainments, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description when taken in conjunction with the accompanying drawings.